When the mollusk Pleurobranchaea is presented with paired food and aversive electric shock, it quickly learns to suppress feeding behavior (passive avoidance conditioning) and withdraw (active avoidance conditioning) from the previously palatable food. The present research is aimed at determining the neurophysiological substrates of this modified form of taste aversion learning. The three specific goals are: (1) to study the physiological changes in feeding command neurons of the brain that accompany and presumably cause the suppression of feeding behavior during taste aversion learning; (2) to study the physiological changes in withdrawal command neurons of the brain that accompany and presumably cause acquisition of withdrawal behavior during taste aversion learning; and (3) to continue neurophysiological analysis of the feeding and withdrawal behaviors, as necessary to the attainment of the first two goals. This research is expected to provide a neurophysiological explanation of these forms of associative learning on the level of single, reidentifiable nerve cells, using a relatively simple invertebrate ("model") nervous system. The research has health-related significance to learning mechanisms and disabilities and to the neural control of movement.